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A long time ago, I had the privilege of flying in 4 engine turboprop airplanes in which the props were 'synched'(blades rotating in same speed) which of course is necessary for smooth thrust.

However, one of the exercises we performed(as a military aircraft) was called a "crash and dash"; we dove as steep as we could until we could get as close as we could to the surface, then pull out of the dive and attempt to 'fly below' any surface radar that may be tracking us.

Every time we did this maneuver it threw the props out of 'synch' and cause the airframe to shudder violently. Anything that was literally not 'nailed down' was thrown through the cockpit or cabin.

My question is, what effect does this have on the airframe and does one of the events require an inspection, or shorten the check cycle?

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    $\begingroup$ It sounds like it may have been the high g-forces which caused the violent shudders and not the props being out of sync (which normally wouldn't cause those kinds of forces). $\endgroup$ – Lnafziger Apr 1 '16 at 2:02
  • $\begingroup$ @Lnafziger I can't say for sure how many G's-I wasn't 'pinned' in my seat and we didn't need G suits. But it definitely threw the props out of synch whenever we attempted it, and the result was a violent 'juddering' (I suppose caused by the acceleration of force against props that were increasingly unable to hold synchronization....? $\endgroup$ – user2479 Apr 1 '16 at 2:50
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    $\begingroup$ It is called hunting and well explained in this answer. It seems that the frequency difference was equal to a structural eigenmode. Looks like shoddy piloting to me. The effect on the airframe: Do it long enough and it will fall apart. $\endgroup$ – Peter Kämpf Apr 1 '16 at 6:23
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    $\begingroup$ I think it's more likely that the wings started fluttering than that the props shook the aircraft. $\endgroup$ – ratchet freak Apr 1 '16 at 9:48
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    $\begingroup$ @PeterKämpf Thank you for that reference-I tried my initial search and I didn't come up with it. The wings of a Lockheed Orion are pretty stiff, and I have seen landings in 45mph crosswinds in them(Adak or Shemya). It is definitely the prop synch that was exacerbated by the steep rate of descent-something hopefully you will never have to do unless you are evading a SAM lock-on, or landing at a 'hot' airstrip. $\endgroup$ – user2479 Apr 1 '16 at 17:27
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I would agree with the other comments, in that I don't think the extreme buffeting could be caused by out-of-sync props. Maybe if you lost a chunk of a blade and you had out-of-balance props, but that's a bit different. Or if one of the props came WAY out-of-sync. But I know nothing of the Orion aerodynamics or engineering.

My twin experience has been more of the light piston and turboprop variety, but I can tell you that out-of-sync props produce a "whah whah whah" sound that will drive you absolutely bonkers. As soon as I was straight and level, I'd match rpms/prop pitch on my engines (or turn on the prop sync) to get rid of the noise. I never wanted to try it long enough, but apparently acoustic oscillation like that can cause serious fatigue. And I would imagine that long-term exposure can't be good for the airframe.

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